Conventionally, a cam shaft of an OHC engine has typically been driven by a chain, but a cogged belt which is lighter in weight and can provide more quiet operation has lately been preferred over a chain. A cogged belt is typically reinforced by steel wires of high tensile strength and stretches very little for its weight, and its longitudinal vibration is not a problem under a normal circumstance. However, in a V-type engine, since the span lengths of the cogged belt between pulleys tend to be long, the longitudinal vibration of the cogged belt may become excessive when the maximum rotational speed of the engine is to be increased, and the strength of the base ends of the teeth of the cogged belt may become a problem.
The motion of a cam shaft drive system is typically coupled with the motion of the valve drive system and is therefore so complex that it is extremely difficult to predict its behavior. Generally, it has been considered as desirable to reduce the moments of inertia of the pulleys as much as possible to the end of raising the natural frequency of the system. And, hitherto, in designing a cam shaft drive system for a V-type OHC engine, it has been customary to use a pair of cam shaft pulleys of an identical configuration and having the least possible masses. However, according to the analyses conducted by the inventors it was found that such an approach is not necessarily appropriate in the case of a cam drive structure for an OHC V-type engine, and on the contrary a favorable result was obtained by making the moment of inertia of one of the cam shaft pulleys greater than that of the other.